This application claims priority of the German patent application 101 40 174.4 which is incorporated by reference herein.
The invention concerns a coordinate measuring stage with interferometric position determination, as well as a coordinate measuring instrument for high-accuracy measurement of the coordinates of the edges of a pattern element on a substrate
Coordinate measuring stages of the kind cited above are used in high-accuracy coordinate measuring instruments of the kind cited above. One such coordinate measuring instrument is described in the article xe2x80x9cMaskenmetrologie mit der LEICA LMS IPRO fxc3xcr die Halbleiterproduktionxe2x80x9d [Mask metrology using the LEICA LMS IPRO for semiconductor production] by K. -D. Rxc3x6th and K. Rinn, Mitteilungen fxc3xcr Wissenschaft und Technik Vol. XI, No. 5, pp. 130-135, October 1997. This measuring instrument is used for high-accuracy measurement of the coordinates of the edges of a pattern element on a substrate, e.g. a mask and a wafer.
The coordinate measuring instrument comprises a measuring stage of the kind cited above which is horizontally displaceable in the X direction and the Y direction. It serves to receive the substrates having features whose edge coordinates are to be measured. In addition, a separate interferometer measurement beam path is associated with each coordinate axis (X, Y) of the measuring stage. Mounted on two mutually perpendicular sides of the measuring stage are measurement mirrors that are located at the ends of the two interferometer measurement beam paths. By means of the two measurement mirrors, the position of the measuring stage can be determined interferometrically.
The coordinate measuring instrument furthermore possesses an incident-light illumination device having an optical axis; an imaging device (for example, a microscope objective); and a detector device (for example, a high-resolution digital camera or a position-sensitive detector) for the imaged features. The measured coordinates of an edge of a feature to be measured are obtained from the interferometrically measured present position of the measuring stage and the distance (relative to the optical axis) of the feature to be measured. The positioning accuracy with which the interferometric measurement of the measuring stage position is acquired therefore directly influences the determination of the coordinates of the edges on the features on the substrates. Because an optimum measurement result requires that optical sensing occur in an accurately defined focal plane (called the Abbxc3xa9{acute over ( )} plane), deviations in vertical running accuracy from that ideal plane also contribute error components to the measurement results.
Coordinate measuring instruments of the aforesaid kind serve for the determination of coordinates with a reproducibility in the range of less than 5 nm. Since this measurement accuracy depends very substantially on the X-Y positioning accuracy and the vertical running accuracy of the measuring stage, the requirements in terms of the construction of the measuring stage are extremely high.
A coordinate measuring stage of the coordinate measuring instrument described above comprises the following elements arranged one above another:
a stationary base part having a linear X guidance element;
above that, a center part, movable slidingly along the linear X guidance element, which has a first drive element associated with it and is rigidly joined to a linear Y guidance element;
above that, an X-Y-positionable stage body which is movable slidingly along the Y guidance element and has a second drive element associated with it.
The X guidance element is configured on the center of the base part, for example as a recessed groove on which the center part is guided. The wide center part is arranged transversely to the X guidance element and forms, with the X guidance element, a movable cross. This stage construction is therefore commonly known to those skilled in the art as a xe2x80x9ccross-slide stage.xe2x80x9d The center part carries the Y guidance element on its center axis, and is supported on the surface of the base part, on either side of the X guidance element, with a total of three support air bearings. The result is a stable three-point support.
The stage body is guided in its motion by the Y guidance element, and spans over the center part. It is supported on the surface of the base part, on either side of the X guidance element, with a total of four support air bearings, i.e. with two support air bearings on each side of the X guidance element. Since a bearing system having four support surfaces is redundant, one of the four support air bearings is configured as a resilient support air bearing in order to compensate for irregularities on the surface of the base part.
Since the coordinate measuring machine must be operated at a constant ambient temperature in order to achieve optimum measurement results, it is set up in a climate-controlled chamber. In individual factories, different ambient temperatures are required in the climate-controlled chambers. The instruments are thus operated, depending on the temperature selected, in a temperature range between 20 and 23xc2x0 Celsius. These temperature differences result in different material elongations at the individual components of the coordinate measuring machine. This in turn changes the air gaps of the air bearings. Some of the guidance air bearings were therefore also designed to be resilient. For example, on each guidance element the guidance air bearings on one side of the guidance element are designed to be resilient, and the guidance air bearings on the opposite side are designed to be rigid.
Both the support air bearings and the guidance air bearings were selected with very small air gaps (on the order of 3 to 4 xcexcm). Adaptation to irregularities of the particular guidance surface is performed by way of the respective resiliently mounted support or guidance air bearings. The narrow air gap heights are highly tolerance-sensitive and must therefore be aligned.
The resilient air bearings have proven to be problematic in practice, however, since they permit a slight tilting and therefore warping of the stage body upon displacement of the measurement stage and with different substrate weights. In addition, they can transfer the vertical orientation inaccuracies of the center part, caused by the irregularity of the base part surface, via the Y guidance element to the stage body. This results in a change in the interferometric position measurement as a function of the X-Y position that is arrived at, and thus has direct repercussions on measurement accuracy.
A further disadvantage of this measurement stage is the fact that transmitted-light illumination of the substrate is not possible, since the center part extends, with the Y-guide, in the center of the stage. The known measurement stage is also suitable only for substrates up to a maximum size of 220 mm. Any enlargement of the individual elements of the measurement stage for the purpose of adaptation to larger substrates would simply aggravate the vibration problem.
It is therefore an object of the present invention to describe a coordinate measuring stage that is suitable for both incident-light and transmitted-light measurements, that is suitable for future even larger and heavier substrates (with diameters of 300 mm and more) and at the same time exhibits greatly improved X-Y running accuracy and improved vertical running accuracy.
This object is achieved by a coordinate measuring stage with interferometric position determination that comprises:
a) a stationary base part having a linear X guidance element;
b) a center part placed above said stationary base part, wherein said center part is movable slidingly along the linear X guidance element, which has a first drive element associated with it and is rigidly joined to a linear Y guidance element;
c) an X-Y-positionable stage body placed above said center part, wherein said X-Y-positionable stage body, which is assigned for reception of a substrate, is movable slidingly along the Y guidance element and has a second drive element associated with it;
d) said center part is arranged in freely suspended fashion over said base part, being supported with its one end on said Y guidance element and with its other end on an additionally arranged support element;
e) said Y guidance element, said support element, and said stage body are supported, slidably and independently of one another, on the surface of said base part; and
f) said base part, said center part, and said stage body each comprise an internally located opening for a transmitted-light region.
A further object of this invention is to describe a coordinate measuring instrument for high-accuracy measurement of the coordinates of the edges of a pattern element on a substrate that is suitable for both incident-light and transmitted-light measurements, that is suitable for future even larger and heavier substrates (with diameters of 300 mm and more) and at the same time exhibits greatly improved X-Y running accuracy and improved vertical running accuracy.
This object is achieved by a coordinate measuring instrument for high-accuracy measurement of the coordinates of an edge of a pattern element on a substrate, having:
a) an X-Y displaceable coordinate measuring stage with interferometric position determination, which comprises a stationary base part having a linear X guidance element; above that, a center part movable slidingly along the linear X guidance element; and above that, an X-Y-positionable stage body, for reception of the substrate, which is movable slidingly along the Y guidance element;
b) and an illumination device having an optical axis, an imaging device, and a detector device for determining the coordinates of said edge to be measured relative to said optical axis of said illumination device;
c) said center part is arranged in freely suspended fashion over said base part, being supported with its one end on said Y guidance element and with its other end on an additionally arranged support element;
d) said Y guidance element, said support element, and said stage body are supported, slidably and independently of one another, on the surface of said base part; and
e) said base part, said center part, and said stage body of said measuring stage each comprise an internally located opening for a transmitted-light region, and said illumination device is provided for incident-light and transmitted-light illumination of the substrate.
To achieve the stated object it was not sufficient to adapt the superimposed elements (base part, center part, and stage body) to the large substrates and merely enlarge them appropriately, since the requisite greatly enlarged displacement range would have made the measuring stage very large and heavy and merely aggravated the vibration problems. That would firstly have created an extremely unstable design and decreased vertical running accuracy. In addition, because of the guide elements required for the X and Y directions, a transmitted-light configuration could not have been realized. A fundamentally different approach to achieving the object thus had to be taken.
The idea of the invention consists substantially in greatly reducing the tolerance chain in the new design. In other words, for example, the sum of the all mechanical tolerances in the Z direction (i.e. the height direction of the stage) had to be decreased. For that purpose, the tolerances in the Z direction for the individual parts connected mechanically to one another had to be calculated during design, taken into account in terms of production engineering, and reduced, so that an extremely good running accuracy for the stage in the X-Y plane was achieved.
Achieving this resulted in a complete change in the mechanical concept of the stage assemblage, departing entirely from the concept of the xe2x80x9ccross-slide stage.xe2x80x9d In the coordinate measuring stage according to the existing art, for example, the center part represented a supporting part and carried the Y guidance element on its longitudinal axis of symmetry. In contrast to this, in the measuring stage according to the present invention the Y guidance element was transformed into the supporting element and the center part into the supported element. This was done by removing the Y guidance element from the axis of the symmetry of the center part and arranging it at the one end of the center part as a supporting part. The other end of the center part was suspended from an additional support element. The center part was thus arranged in suspended fashion, being supported by the Y guidance element and the additionally arranged support element.
This suspended center part also offered for the first time the possibility of achieving a transmitted-light configuration, since the Y guidance element is always arranged far away from the center of the center part and thus outside the center of the measuring stage. The base part, center part, and stage body were therefore equipped with frame-shaped openings in order thereby to create a transmitted-light region.
The Y guidance element and the support element rest with support air bearings on the surface of the base part. The Y guidance element was dimensioned very long, and the support element dimensioned as short as possible. In addition, the center part in the X direction (and thus the spacing between the Y guidance element and the support element) was made as large as possible, so that the air bearings could be arranged at a wide spacing from one another. The result was to achieve a high level of running stability for the measuring stage, with no tendency to tilt. The long center part at the same time offered the ability to make the frame-shaped opening as large as possible in the X direction.
In the coordinate measuring stage of the existing art, a complex tolerance chain for the individual components of the coordinating measuring stage had to be taken into account. For example, the surface flatness values of the stage body, center part, and base part, the dimensional accuracy of the Y guidance element on the center part, the tolerances of the respective support air bearings (flying heights, mounting surface flatness values), and the alignment accuracy of the guidance air bearings, influenced the tolerance calculation. Several tolerances (three or more, as a rule) in terms of shape, position, and dimension had to be complied with for each individual component. Despite that, the air gaps of the guidance air bearings still had to be adjusted individually. Each tolerance-sensitive part also resulted in higher production costs.
Instead of this, in the coordinate measuring stage according to the present invention the reference plane for tolerance calculation is the plane surface of the base part. The stage body is supported slidingly on the base part by means of unsprung support air bearings. Its tolerances are referred to the same reference plane, namely the plane surface of the base part. The individual components of the coordinate measuring stage according to the present invention, for example the center part, stage body, Y guidance element, and Y-guide, need to comply with only one or two tolerances in terms of shape, position, and dimension. For example, only the underside of the stage body, on which the support air bearings are arranged, needs to be configured with accurate flatness. The flatness of the surface of the base part determines the entire vertical running accuracy (deviations from the ideal Z plane) of the coordinate measuring stage according to the present invention.
The air gap spacings of the support air bearings and the guidance air bearings are predefined by the adjacent components, so that the air gaps of the support air bearings and guidance air bearings no longer need to be adjusted, and the structure as a whole is stiffer and therefore less susceptible to vibration than in the case of the known coordinate measuring stage. Running accuracy and positioning accuracy are thus substantially improved with the measuring stage according to the present invention. This in turn improves the measuring accuracy of the coordinate measuring instrument in which the coordinate measuring stage is arranged for reception of the substrate to be measured.
The base part is usually made of granite. For reasons of vibration damping, a large granite block mounted on vibration dampers is used. The Y guidance element and support element, as well as the stage body, are supported via their plane undersides, by means of support air bearings, on the surface of the base part (granite). All the support air bearings are selected to have the same thickness, the absolute thickness tolerance of the support air bearings being on the order of approx. 2 xcexcm. This is advantageous in terms of bearing maintenance and service, since no alignment work is necessary when replacing a damaged bearing. Since the support air bearings used are all identical, parts count is reduced and individually manufactured bearings are not necessary. All that is necessary for the support air bearings is a sufficient flying height (=air layer).
Resilient mounting was entirely dispensed with in the guidance air bearings as well. The guidance air bearings are independent of thickness tolerance. Only the bolt-on surface and the running surface from which air emerges need to lie exactly in one plane. This was achieved by common lapping of the bolt-on surface and running surface. Alignment of the guidance air bearings is superfluous, since the air gap width is predefined by the adjacent components.
An embodiment of the coordinate measuring stage in which the X guidance system and Y guidance system are of temperature-compensated configuration is particularly advantageous. To achieve this, those components of the coordinate measuring stage on which the guidance bearings for the X guidance element and Y guidance element are mounted are temperature-compensated with respect to the X guidance element and Y guidance element, respectively. This is important because the width of the components of the coordinate measuring stage on which the guidance bearings are mounted determines the spacing of the air bearing installation surfaces that face them, and thus directly influences the width of the guiding air layers that are produced (and hence the guidance properties).
For temperature compensation of the components on which the guidance bearings are mounted, they are selected to be of the same material as the material of the recessed grooves or raised ribs (depending on embodiment) of the X guidance element or Y guidance element at which the guidance air bearings generate guidance with the air gaps that they produce. To ensure additionally that the X and Y guidance systems do not have different expansion characteristics, the two guidance elements (for X and Y) were also selected to be of the same material. Since the base part and thus the X guidance element are made of granite, it followed that the Y guidance element was also made of granite.
The components on which the guidance bearings are mounted were therefore also fabricated from granite. In addition, these components are dimensioned so that they are narrower than the grooves themselves only by an amount equal to the two guiding air layers, or wider than the ribs themselves only by an amount equal to the two guiding air layers.
The result of this is that these components on which the guidance bearings are mounted exhibit exactly the same coefficients of expansion as the grooves or ribs (depending on embodiment) of the X and Y guidance elements. This ensures that even with differences in the ambient temperature of the coordinate measuring stage, the air gaps of the guidance air bearings defined for guidance purposes always remain unchanged. This results in a definite increase in guidance accuracy as compared to previously known guidance system designs.
In an advantageous embodiment of the measuring stage, the vertical running accuracy was improved even further by the fact that the drive elements for the X direction and Y direction were arranged in stationary fashion relative to the base part. For that purpose, energy transfer to the center part movable in the X direction, and to the stage body movable in the Y direction, is effected e.g. by means of at least one push bar in each case, which is connected via a friction coupling to the respective motor. The result of this is that the vertical running accuracy and positioning accuracy of the coordinate measuring stage are constant over the entire displacement range.
This is a distinct advantage over the previously known coordinate measuring stages in which one of the motors was co-moved. In these stages, the weight of the co-moving motor itself resulted, even when the stage body was not loaded, in deformations of the entire measuring stage and thus in a degradation in positioning accuracy as a function of X-Y position. Cable delivery to the motors is also simplified, since the cables no longer need also to be moved; this decreases both wear on the cables and impacts on the coordinate measuring stage. Since the positioning error with the previously known measuring stage was not constant but instead depended on the measurement location and additionally on the weight of the substrate placed on it, the positioning error also could not be eliminated computationally from the measured coordinates of an edge.
The coordinate measuring stage according to the present invention having the stationary drive elements, on the other hand, exhibits a positioning error value that is independent of displacement travel for all attainable X-Y positions and can be accounted for computationally and instrumentally. In addition, the weight that is put in place in each case is kept constant. For that purpose, each substrate that is to be measured in transmitted light is placed in a specifically allocated frame and placed on the stage body together with the frame. For the various substrates, the sum of the weight of the particular substrate and the weight of the associated frame is kept constant. Upon calculation of the coordinates of features to be measured, an error correction can thus be performed for this positioning error value as well.